Reduce gem size by excluding test files

[yuri-zubov]

This pull request updates the *.gemspec file to optimize the gem package size and structure

$ gem build -o before.tar.gz

$ git switch reduce-gem-size

$ gem build -o after.tar.gz

$ du -sh before.tar.gz after.tar.gz
28K  before.tar.gz
24K  after.tar.gz

Metric	Before	After	Saved
Size	28K	24K	−4K (⏷ −14.29%)

whitch files was deleted?

 data
-├── .github
-│   └── workflows
-│       └── ruby.yml
-├── .gitignore
-├── Appraisals
-├── Gemfile
 ├── lib
 │   ├── sidekiq-benchmark
 │   │   ├── testing.rb
 │   │   ├── version.rb
 │   │   ├── web.rb
 │   │   └── worker.rb
 │   └── sidekiq-benchmark.rb
 ├── LICENSE.txt
-├── Rakefile
 ├── README.md
-├── sidekiq-benchmark.gemspec
-├── test
-│   ├── lib
-│   │   ├── testing.rb
-│   │   ├── web_test.rb
-│   │   └── worker_test.rb
-│   └── test_helper.rb
 └── web
     ├── assets
     │   └── javascripts
     │       └── chartkick.js
     └── views
         └── benchmarks.erb

ps: you can see on rails repo